The petroleum distillate fraction that contains C.sub.4 to C.sub.7 hydrocarbons is relatively low in octane because it contains substantial amounts of low octane, normal paraffins. For example, normal C.sub.5 has a blending RON of 62 and normal C.sub.6 has a blending RON of 19 (blending RON will be hereinafter called "RON"). However, when these paraffins are isomerized to form branched paraffins, their RON increases dramatically. For example, isopentane (2-methylbutane) has a RON of 99 and isohexane (2-methylpentane) has a RON of 83. Generally, the RON will increase with even higher branching (e.g., 2,2-dimethylbutane has a RON of 89).
Several catalysts have been used to isomerize these lower octane paraffins into the branched, higher octane paraffins. Examples are shown in U.S. Pat. No. 4,374,296 issued Feb. 15, 1983 to Haag et al.; U.S. Pat. No. 3,432,568 issued Mar. 11, 1969 to Miale et al.; U.S. Pat. No. 3,673,267 issued Jun. 27, 1972 to Chen et al.; and U.S. Pat. No. 4,665,272 issued May 12, 1987 to Bakas et al. Haag et al. disclose isomerizing paraffins using intermediate pore zeolites, which have a constraint index between 1 and 12. Miale et al. described hydroisomerizing saturated aliphatic and cyclic hydrocarbons by contacting them with a dual functional catalyst comprising mordenite and a catalytic metal. Chen et al. describe a process for isomerizing paraffins using mordenite having a silica to alumina ratio between 20:1 and 60:1. Bakas et al. disclose isomerizing paraffins using a crystalline aluminosilicate having a catalytic metal.
However, even though these prior art catalysts are useful, there remains a need for a new catalyst that is: (1) highly active; (2) highly selective for producing high octane liquid product; and (3) sulfur tolerant. That need is satisfied by the invention that is detailed below.